Cytotoxic Activity of CD4(+) T Cells against Autologous Tumor Cells

The (51)Cr‐release assay is mostly applied to detecting the cytotoxic activity of CD8(+) T cells, and little is known about the activity of CD4(+) T cells. Therefore, the correlation between the cytotoxic activity of CD4(+) or CD8(+) T cells and the incubation period with autologous tumor cells was analyzed by two methods. The incubation periods were 4 and 20 h (4 h and 20 h assay) for the (51)Cr‐release assay. Eight pairs of tumor cells and T cells were assayed. T cells were fractionated into CD4(+) and CD8(+) T cells by using magnetic beads and panning methods, and those cells were activated by culture with recombinant interleukin‐2 and immobilized anti‐CD3 monoclonal antibody. In 6 out of 8 cases, no cytotoxic activity of CD4(+) T cells was detected by the 4 h assay, whereas cytotoxic activity was detected in all cases in the 20 h assay. The cytotoxic activities in 20 h assay of CD4(+) T cells were increased 67‐fold in comparison with the activities in 4 h assay (range: 5–197). In the case of CD8(+) T cells, cytotoxic activities were detected in 6 out of 8 cases in the 4 h assay. The lytic unit ratio of CD4(+) and CD8(+) T cells was calculated as 1.5 in the 20 h assay (range: 0.2‐>7.2) versus 0.4 in the 4 h assay (range: < 0.1–1.3). Cytotoxic activities in colorimetric assay using Crystal Violet with a 24 h incubation were similar to those in the 20 h (51)Cr‐release assay in all eight cases. These results indicate that CD4(+) T cells have cytotoxic activity as strong as that of CD8(+) T cells towards autologous tumor cells.